A new polypeptide derivative, poly(N-ethoxycarbonyl-L-valine) (2) was prepared for the first time by the cationic ring-opening polymerization of (S)-2-ethoxy-4-isopropyl-5(4H)-oxazolone (1). The polymerization proceeded with a cationic initiator, methyl trifluoromethanesulfonate, BF(3)OEt(2), or trifluoromethanesulfonic acid, in an aprotic non-nucleophilic solvent such as toluene, chloroform, or nitromethane. The molecular weight of the resulting 2 was independent of the feed ratio of monomer to initiator but depended on the polymerization temperature. The number average molecular weight of 2 prepared at -40 degrees C was 11000. The resulting polymer 2 showed high solubility in organic solvents, especially in halogenated hydrocarbons.